Loudspeakers generally comprise a frame, a motor structure, a diaphragm, a lower suspension or spider and a surround or upper suspension. In one type of speaker, the motor structure includes a permanent magnet sandwiched between a top plate and a back plate, with a pole piece centrally mounted on the back plate so that both the top plate and magnet are concentrically disposed about the pole piece. A magnetic gap is formed between the pole piece and top plate within which a voice coil is axially movable. Preferably, the voice coil consists of a hollow, cylindrical-shaped former having an inner surface and an outer surface which mounts a winding of wire.
The voice coil is mounted within the magnetic gap by the upper and lower suspensions and the diaphragm. One end of the diaphragm is connected to the surround, which, in turn, is mounted to the upper end of the frame. The spider is connected at one end to the frame at a point between its upper and lower ends. The free ends of the diaphragm and spider are mounted to the outer surface of the former of the voice coil and support it for axial movement within the magnetic gap.
In the course of operation of speakers of the type described above, electrical energy is supplied to the voice coil causing it to axially move within the magnetic gap. The voice coil, diaphragm, surround and spider collectively form a “moving assembly” which reciprocates as a unit with the excursion of the voice coil.
To achieve highest magnetic efficiency in a loudspeaker, the magnetic gap must have a small area with tight tolerances. If the voice coil deviates appreciably from its axial path within the magnetic gap, the voice coil can contact the pole piece or top plate of the motor structure causing significant damage to the speaker. The spider permits free travel of the moving assembly while maintaining it in a centered position with respect to the magnetic gap. During excursion of the voice coil, the spider “tightens up” or stretches to an outer limit of displacement thus limiting the overall extent of excursion permitted and protecting the moving assembly from contacting other parts of the loudspeaker. Additionally, the spider is primarily responsible for exerting a restoring force on the voice coil so that it moves toward the “rest” position within the magnetic gap, i.e. where the voice coil is located when the speaker is not operating, which has a direct impact on distortion performance.
In most speaker designs, spiders are geometrically formed pieces of a cloth-like material. A resin is applied to the material before the forming operation in an attempt to ensure that the final shape of the spider is maintained. The shape of the spider is important because it determines the centering and excursion control performance of the spider.
The spider is typically glued along its inner diameter to the outer surface of the former of the voice coil. A generally flat ring section or foot is formed along the outer diameter of the spider, which, in most speaker designs, is glued to the frame. Typically, these gluing operations take place within the loudspeaker frame as an aid to fixturing the various parts in the proper spatial orientation. The spider-to-frame glue joint is especially critical because it provides the reference point for centering and axial movement of the voice coil as well as the rest of the moving assembly.
For various reasons, but most often as a result of shipping and handling, the spiders which are provided by manufacturers for assembly in loudspeakers may not be flat. That is, the attachment foot along the outer diameter of the spider often does not lie in the same plane. This geometric deformation of the spider foot not only compromises performance of the spider, but it can adversely affect the glue joint between the spider foot and frame. While some areas of the foot contact the frame and can be securely glued in place, other areas of the foot may be slightly separated from the frame resulting in a less glue-foot contact than is desired.
The problem of attaching a geometrically deformed spider foot to the speaker frame cannot be satisfactorily overcome by merely pressing the foot down onto the frame during assembly. Typically, a bead of glue is first applied to the frame and then the spider is brought into contact with the glue. Due to the nature of the glues used for the spider foot-frame joint connection, applying pressure to the foot to force it down onto the frame during assembly would cause the glue on the frame to “squirt out” from between the spider foot and frame. The displaced glue may cover the spider, thus comprising its performance, or spill onto other areas of the speaker. As a result, a relatively thick glue bead is typically used to try to ensure all areas of the spider foot are bonded to the frame. However, the glue bead itself can create geometrically uneven areas along the spider foot, particularly if the glue bead is applied unevenly to the frame.
Another problem with prior art speakers involves failure of the joint connection between the spider foot and frame. When the moving assembly approaches the limits of its excursion, a force is exerted on the spider-to-frame glue joint as the spider acts to pull the moving assembly back toward the neutral position. Since the spider is made of a cloth-like material, this glue joint is susceptible to “peel.” Starting at the inner diameter of the frame glue joint, it has been found that the spider can gradually peel away from the surface of the frame particularly in high excursion speakers such as subwoofers. If the spider detaches in one area, it's just a matter of time until the loudspeaker fails.